I would start by thinking about what role friends play in your own life. You'll be able to write about it much more easily if the points you are making are ones you, yourself, feel and believe.
For me, friends are important for a variety of reasons:
- They give you people with whom to talk about things that are bothering you.
- They allow you to feel good about yourself -- to feel that there are people who like you.
- They keep you from being too self-centered because you have to think about their needs as well.
I'm sure other people will have other ideas, but I hope these will be of help. Good luck!